The present invention relates to a system and method for employing dispersion management techniques in an optical transmission system and in particular to achieving dispersion optimization by conducting real-time measurements of demodulated received signals from an optical transmission line.
In optical transmission systems, especially wavelength division multiplexed (WDM) long-haul and ultra long-haul optical transmission systems, a major design issue includes fiber dispersion (or chromatic dispersion) management. The maximum bit rate of WDM systems is limited by, among other things, the group-velocity dispersion (GVD). Dispersion is commonly defined as the separation of a beam into its various wavelength components. In an optical transmission system, especially WDM systems, dispersion penalties occur because the differing wavelengths propagate at differing speeds and a receiver must account for such differences or the received signal will be diminished if not undetectable. Dispersion optimization is also an essential part of high bit-rate system installation. Managing dispersion is thus important in optical transmission systems, especially WDM systems.
One of the major challenges in WDM and wideband dense WDM (DWDM) long-haul transmission systems, in terms of dispersion management strategy, is to optimize pre and post dispersion compensation in order to correct for accumulated dispersion at the receiver. The ratio of pre/post dispersion serves to pre-bias the transmitted pulse to mitigate the non-linear effects during transmission and to effectively reconstruct the transmitted pulse at the receiver. That is, transmission performance of high bit-rate systems strongly depends on residual dispersion as well as the ratio of dispersion pre-compensation to total dispersion compensation (i.e., dispersion split ratio).
Most of the research surrounding dispersion management studies is generated through modeling and experimentally verified through loop experiments before being transferred to a fielded system. Currently, in experimental and field deployed systems, the exhaustive search for the optimum dispersion compensation values and ratios involves the arduous task of setting the likely dispersion, measuring the Q-factor (or bit error rate [BER]) and observing the received eye diagram. Then, if not optimum, the tester must change the dispersion setting, measure the Q-factor again and observe the received eye diagram again and again until optimum dispersion is achieved for a given wavelength channel. Furthermore, this must be done for each wavelength in the transmission line.
Known processes are slow and cumbersome. For example, in conventional systems, after establishing an optical connection, the dispersion is optimized using a kind of channel performance optimization, where pre/post compensation values are changed to achieve best performance. It may take several iterations before the optimum pre/post dispersion ratios are found for a given deployed system. Then, as the system ages, the optimum dispersion may change. Testing for optimum dispersion may be needed again to compensate for the aged transmission system and the same time consuming tests would have to be employed.
Known methods for optimizing dispersion use BER or Q-factor measurements, which, as mentioned above, are usually time consuming and the metric requires numerous adjustments. In addition, the error performance of the recovered digital signal must be measured, which makes the task of determining optimum dispersion dependent upon the payload of the transmission system. This includes being dependent upon the bit rate and modulation format(s) of the transmitted signals. Conventional BER measurements usually require the transmission of a particular test signal, such as (pseudo random bit sequence (PRBS), and thus cannot be carried out on live traffic. Conventional BER measurements also require frame synchronization with the received signal, necessitating more hardware. In some systems employing forward error correction (FEC), FEC framing and decoding may also be necessary to carry out BER measurements. Other known systems require clock and data recovery and expensive BER test sets (BERT).
Dispersion optimization of a system at installation and commissioning is a laborious process that requires changes to dispersion compensation at both the transmit end and the receive end because the balance of pre-compensation to post-compensation also must be optimized. This process must be carried out for every channel in a multichannel WDM or DWDM transmission system, leading to an inordinate amount of time spent during the commissioning of the system.